.\"
.\" Copyright (c) 2010-2011 The FreeBSD Foundation
.\"
.\" This documentation was written at the Centre for Advanced Internet
.\" Architectures, Swinburne University of Technology, Melbourne, Australia by
.\" David Hayes under sponsorship from the FreeBSD Foundation.
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.Dd May 13, 2021
.Dt CC_HD 4
.Os
.Sh NAME
.Nm cc_hd
.Nd HD Congestion Control Algorithm
.Sh DESCRIPTION
The HD congestion control algorithm is an implementation of the Hamilton
Institute's delay-based congestion control which aims to keep network queuing
delays below a particular threshold (queue_threshold).
.Pp
HD probabilistically reduces the congestion window (cwnd) based on its estimate
of the network queuing delay.
The probability of reducing cwnd is zero at hd_qmin or less, rising to a maximum
at queue_threshold, and then back to zero at the maximum queuing delay.
.Pp
Loss-based congestion control algorithms such as NewReno probe for network
capacity by filling queues until there is a packet loss.
HD competes with loss-based congestion control algorithms by allowing its
probability of reducing cwnd to drop from a maximum at queue_threshold to be
zero at the maximum queuing delay.
This has been shown to work well when the bottleneck link is highly multiplexed.
.Sh MIB Variables
The algorithm exposes the following tunable variables in the
.Va net.inet.tcp.cc.hd
branch of the
.Xr sysctl 3
MIB:
.Bl -tag -width ".Va queue_threshold"
.It Va queue_threshold
Queueing congestion threshold (qth) in ticks.
Default is 20.
.It Va pmax
Per packet maximum backoff probability as a percentage.
Default is 5.
.It Va qmin
Minimum queuing delay threshold (qmin) in ticks.
Default is 5.
.El
.Sh SEE ALSO
.Xr cc_cdg 4 ,
.Xr cc_chd 4 ,
.Xr cc_cubic 4 ,
.Xr cc_dctcp 4 ,
.Xr cc_htcp 4 ,
.Xr cc_newreno 4 ,
.Xr cc_vegas 4 ,
.Xr h_ertt 4 ,
.Xr mod_cc 4 ,
.Xr tcp 4 ,
.Xr khelp 9 ,
.Xr mod_cc 9
.Rs
.%A "L. Budzisz"
.%A "R. Stanojevic"
.%A "R. Shorten"
.%A "F. Baker"
.%T "A strategy for fair coexistence of loss and delay-based congestion control algorithms"
.%J "IEEE Commun. Lett."
.%D "Jul 2009"
.%V "13"
.%N "7"
.%P "555-557"
.Re
.Sh ACKNOWLEDGEMENTS
Development and testing of this software were made possible in part by grants
from the FreeBSD Foundation and Cisco University Research Program Fund at
Community Foundation Silicon Valley.
.Sh FUTURE WORK
The Hamilton Institute have recently made some improvements to the algorithm
implemented by this module and have called it Coexistent-TCP (C-TCP).
The improvements should be evaluated and potentially incorporated into this
module.
.Sh HISTORY
The
.Nm
congestion control module first appeared in
.Fx 9.0 .
.Pp
The module was first released in 2010 by David Hayes whilst working on the
NewTCP research project at Swinburne University of Technology's Centre for
Advanced Internet Architectures, Melbourne, Australia.
More details are available at:
.Pp
http://caia.swin.edu.au/urp/newtcp/
.Sh AUTHORS
.An -nosplit
The
.Nm
congestion control module and this manual page were written by
.An David Hayes Aq Mt david.hayes@ieee.org .
